Anomalous high energy waterfall like electronic structure in 5 d transition metal oxide sr2iro4 with a strong spin orbit coupling

Anomalous High Energy Waterfall Like Electronic Structure in 5 d Transition Metal Oxide Sr2IrO4 with a Strong Spin Orbit Coupling 1Scientific RepoRts | 5 13036 | DOi 10 1038/srep13036 www nature com/s[.]

Like Electronic Structure in 5 d

a Strong Spin-Orbit Coupling

Yan Liu 1 , Li Yu 1 , Xiaowen Jia 1 , Jianzhou Zhao 1 , Hongming Weng 1,2 , Yingying Peng 1 , Chaoyu Chen 1 , Zhuojin Xie 1 , Daixiang Mou 1 , Junfeng He 1 , Xu Liu 1 , Ya Feng 1 , Hemian Yi 1 , Lin Zhao 1 , Guodong Liu 1 , Shaolong He 1 , Xiaoli Dong 1 , Jun Zhang 1 , Zuyan Xu 3 ,

Chuangtian Chen 3 , Gang Cao 4 , Xi Dai 1,2 , Zhong Fang 1,2 & X J Zhou 1,2

The low energy electronic structure of Sr 2 IrO 4 has been well studied and understood in terms of

an effective Jeff = 1/2 Mott insulator model However, little work has been done in studying its high energy electronic behaviors Here we report a new observation of the anomalous high energy electronic structure in Sr 2 IrO 4 By taking high-resolution angle-resolved photoemission measurements

on Sr 2 IrO 4 over a wide energy range, we have revealed for the first time that the high energy electronic structures show unusual nearly-vertical bands that extend over a large energy range Such anomalous high energy behaviors resemble the high energy waterfall features observed in the cuprate superconductors While strong electron correlation plays an important role in producing high energy waterfall features in the cuprate superconductors, the revelation of the high energy anomalies in Sr 2 IrO 4 , which exhibits strong spin-orbit coupling and a moderate electron correlation, points to an unknown and novel route in generating exotic electronic excitations.

The transition metal oxides exhibit rich exotic physical properties such as high temperature supercon-ductivity and colossal magnetoresistance that have become a central theme of modern condensed matter

5d transition metal oxides, the electron correlation is expected to become less strong due to the more

1 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 2 Collaborative Innovation Center of Quantum Matter, Beijing, China 3 Technical Institute

of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China 4 Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506 Correspondence and requests for materials should be addressed to X.J.Z (email: XJZhou@aphy.iphy.ac.cn) or L.Y (email: li.yu@iphy.ac.cn)

received: 08 April 2015

Accepted: 16 July 2015

Published: 12 August 2015

that has been reached is Sr2IrO4 being a typical system where the Mott- and slater-type behaviors coexist

structure, magnetic structure, and even possible high temperature superconductivity that is predicted in doped Sr2IrO423,24

Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool to directly probe the low

little work has been done in studying its high energy electronic behaviors In this paper, we report the

bands cannot be understood in terms of the band structure calculations; they cannot be understood

observations point to the significant role of the strong spin-orbit coupling, together with a moderate

electron correlation, in giving rise to new high energy excitations in the 5d transition metal oxides.

weight is present at the Fermi level (not shown in Fig.  1), consistent with the insulating nature of

X(π, 0) and its equivalent locations (Fig. 1a) Further increase of the binding energy to 0.4 eV results in

the enlargement of the spot into a square-shape and the emergence of spectral weight near the Γ (0, 0) point (Fig. 1b) When the binding energy increases to 0.8 eV, the strong spectral weight near X points vanishes with a formation of a few disconnected patches around X, while the spectral weight near Γ exhibits a petal-like shape with four leaves (Fig.  1c) The measured constant energy contours at low

at an intermediate binding energy (e.g., 0.4 eV) are also consistent with the band structure calculations

band structure calculations lays a foundation for our following investigation of high binding energy electronic structure in Sr2IrO4

shows band structure along two high-symmetry momentum cuts covering a large energy range till

~6 eV: one cut is across Γ (Fig.  2a–d), the other is across X (Fig.  2e–h) (for more momentum cuts, see Fig S1, S2 and S3 in Supplementary Materials) Here we show both the original data (Fig.  2a,e), and their corresponding momentum-(Fig.  2b,f) and energy-second-derivative (Fig.  2c,g) images The second-derivative images help to highlight the band structure more clearly although many features are already clear in the original data Since momentum-second derivative image may miss the flat hori-zontal bands while the energy-second derivative image may miss the vertical bands, the energy- and momentum-second-derivative images are complementary to each other to provide a full picture As

represent constant energy contours of the spectral weight distribution for Sr2IrO4 measured at ~20 K at different binding energies (EB) of 0.2 eV, 0.4 eV, and 0.8 eV, respectively (d) is the calculated constant energy

contour at a binding energy of ~0.2 eV by including on-site Coulomb repulsion and spin-orbit coupling7 The orange lines denote the antiferromagnetic Brillouin zone boundary for the IrO2 plane

the electronic structure becomes quite unusual First, the momentum-second derivative images and energy-second-derivative images give rather different band structures for both the Γ and X

is present even beyond 3 eV up to ~6 eV (Fig. 2b) Note that these features are not due to the artifact

of the momentum second-derivative image because they are already clear in the original data (Fig. 2a) Such features can also be identified clearly in the momentum distribution curves (MDCs) where the

in Fig. 2f) are observed up to 4 eV, and another set of vertical bands are seen even up to 6.5 eV (Fig. 2f)

We note that such a high energy band anomaly was not revealed before because the previous ARPES

some indications of the high energy waterfall-like features appear to be present in a recent ARPES study

Figure 3 shows the detailed momentum evolution of the high energy electronic structure: one is near the

Fig. 2, nearly vertical bands are observed in the momentum-second-derivative images (Fig. 3c,g) in both cases for different momentum cuts Furthermore, the constant energy contours exhibit dramatic evolu-tion with the binding energy (Fig. 3a,e) The spectral weight distribuevolu-tion around the Γ point (Fig. 3a) changes from a pocket centered at Γ at a binding energy of 0.4 eV, to butterfly-shaped at 0.6 eV and 0.8 eV, to big-X-shaped at 1.2 eV and to dumbbell-shaped at 2.0 eV and 2.4 eV It is interesting to note that the spectral weight distribution shows discrete four strong spots at 0.6 eV and 0.8 eV, other than a continuous contour From Fig. 3c,d, it becomes clear that the drastic spectral distribution change with the

It is also clear from Fig. 3b that, moving away from the cut across Γ (cut 1), the vase-shaped band and vertical structure persist for the cuts 2 and 3 The same is true for the X point constant energy contours (Fig. 3e) and the momentum-dependent band structures (Fig. 3f–h) First, the constant energy contours near X also exhibit an obvious evolution with the binding energy (Fig. 3e) Second, the vertical bands are present over a large area of momentum space near X (Fig. 3g)

Figure 2 Typical band structures of Sr 2 IrO 4 along high-symmetry cuts in a large energy range (a)

Original photoemission image of Sr2IrO4 measured along a high-symmetry cut across Γ ; the location of

the cut is shown as a solid blue line in the inset (b,c) are corresponding momentum-second-derivative and energy-second-derivative images of (a), respectively (d) Momentum distribution curves (MDCs) at different binding energies obtained from (a) (e) Original photoemission image measured along a high-symmetry cut across X; the location of the cut is shown as a solid blue line in the inset (f,g) are corresponding momentum-second-derivative and energy-second-derivative images of (e), respectively (h) MDCs at different binding energies obtained from (e).

Figure  4 summarizes the band structure of Sr2IrO4 measured along three typical high-symmetry

(Fig. 4a,b) In the calculated band structure (Fig. 4a), the electronic states between the Fermi level and

binding energy the contribution is mainly from the oxygen p orbitals (yellow lines in Fig. 4a) In

binding energy These two bands show good agreement with the band structure calculations (Fig. 4a) and

image (Fig. 4d) can find some good correspondence in the calculated band structure (Fig. 4a) The most dramatic difference between the measurements and calculations lies in the binding energy region above

1 eV As seen in Fig. 4a, a couple of energy bands from Iridium are expected from the band structure calculations within the energy range of 1 ~ 3 eV but are not observed in the measured data (Fig.  4d)

energy range that are completely absent in the calculated band structure (Fig. 4a) The same is for the

3 ~ 6 eV binding energy range where some vertical bands are observed (Fig. 4e) but are not present in the calculated band structure at all (Fig. 4a)

Further inspection of the measured band structure indicates that the new nearly-vertical high energy

which summarizes all the observed bands on top of the original measured data One can see that the

Figure 3 Momentum dependence of the band structures around Γ and X regions (a) Constant energy

contours around Γ point at different binding energies from 0.4 eV (top panel) to 0.6, 0.8, 1.2, 2.0 and 2.4 eV

(bottom panel) (b) Original photoemission images measured along different momentum cuts around Γ The location of the momentum cuts are shown as red lines in the top panel of (a) (c,d) are corresponding momentum-second-derivative and energy-second-derivative images of (b), respectively (e) Constant energy

contours around X point at different binding energies from 0.2 eV (top panel) to 0.4, 0.6, 0.8, 1.2 and 1.8 eV

(bottom panel) (f) Original photoemission images measured along different momentum cuts around X The location of the momentum cuts are shown as red lines in the top panel of (e) (g,h) are corresponding momentum-second-derivative and energy-second-derivative images of (f), respectively.

Jeff = 3/2 because of the strong spin-orbit coupling7 It has been shown that the β0 band is predominantly

feature appearing for all orbitals over a rather high energy scale

To the best of our knowledge, such unusual high energy waterfall-like electronic structures are

it implies nearly infinite electron velocity if interpreted literally in the conventional band structure pic-ture This is reminiscent to the high energy waterfall feature observed in the high temperature cuprate

For a conventional metal, the energy- and momentum-second-derivative images are supposed to produce similar band structure The dichotomy between them already points to an exotic behavior and the effect

of strong correlation Second, nearly vertical bands are observed in the momentum-second-derivative

be used to compare and contrast with the cuprates in order to understand the origin of the high energy anomaly In the cuprate superconductors, the high energy anomalous band has attracted extensive

prime candidate for the anomalous high energy behavior can be simply an intrinsic property of a strong

paramag-nons29,40,42,49, and other spin and charge excitations53 It can also be due to other novel effects such as the

5d transition metal oxides than that in their 3d counterparts (~20 meV), reaching a comparable energy

Figure 4 Calculated and measured overall band structure of Sr 2 IrO 4 (a) Band structure of Sr2IrO4

by DMFT calculations along high symmetry line in the first Brillouin zone The white lines are the LDA + DMFT calculation on Iridium’s t2g orbitals while the yellow lines are the LDA calculation on Oxygen

p orbitals (b) Calculated density-of-states for the Jeff = 3/2 and Jeff = 1/2 states, and the total density-of-states

of the Iridium orbitals (c) Overall measured original photoemission image of Sr2IrO4 along high-symmetry

cuts The observed bands are overlaid on top of the original data (d,e) are corresponding energy-second-derivative and momentum-second-energy-second-derivative images of (c), respectively The black and red lines are guides

to the eye for the bands that can be resolved α1′ , α2′ and β1′ bands are equivalent bands to the α1, α2, and

β1 bands along other symmetry cuts

moderate electron correlation and strong spin-orbital coupling60 Exotic quasiparticles like a composite

further theoretical and experimental investigations

In summary, our ARPES measurements over a wide energy window have revealed for the first time a

feature observed in high temperature cuprate superconductors While the low energy electron excitations

existing band structure calculations Different from the cuprate superconductors where strong electron

provides a new scenario that high energy anomaly can occur in a system with moderate or weak elec-tron correlation and selec-trong spin-orbit coupling We hope these experimental observations can stimulate

and the high energy anomaly in other materials in general

Methods

pho-toemission measurements were carried out on our lab system equipped with a Scienta R4000 electron

hυ = 21.218 eV (helium I) The overall energy resolution was set at 20 meV The angular resolution is

~0.3 degree The Fermi level is referenced by measuring on a clean polycrystalline gold that is electrically

connected to the sample The sample was cleaved in situ and measured at ~20 K in ultra-high vacuum

for several times and the results are reproducible

References

1 Imada, M et al Metal-insulator transitions Rev Mod Phys 70, 1039 (1998).

2 Lee, P A et al Doping a Mott insulator: Physics of high-temperature superconductivity Rev Mod Phys 78, 17 (2006).

3 Mattheiss, L F Band structure and Fermi surface of ReO 3 Phys Rev 181, 987 (1969).

4 Mattheiss, L F Electronic structure of RuO 2 , OsO 2 , and IrO 2 Phys Rev B 13, 2433 (1976).

5 Crawford, M K et al Structural and magnetic studies of Sr2 IrO 4 Phys Rev B 49, 9198 (1994).

6 Cao, G et al Weak ferromagnetism, metal-to-nonmetal transition, and negative differential resistivity in single-crystal Sr2 IrO 4

Phys Rev B 57, 11039(R) (1998).

7 Kim, B J et al Novel J eff = 1/2 Mott state induced by relativistic spin-orbit coupling in Sr2IrO4 Phys Rev Lett 101, 076402

(2008).

8 Fujiyama, S et al Two-dimensional Heisenberg behavior of J eff = 1/2 isospins in the paramagnetic state of the spin-orbital Mott insulator Sr 2 IrO 4 Phys Rev Lett 108, 247212 (2012).

9 Arita, R et al Ab initio studies on the interplay between spin-orbit interaction and Coulomb correlation in Sr2 IrO 4 and Ba 2 IrO 4

Phys Rev Lett 108, 086403 (2012).

10 Moon, S J et al Dimensionality-controlled insulator-metal transition and correlated metallic state in 5d transition metal oxides

21 Yamasaki, A et al Bulk nature of layered perovskite Iridates beyond the Mott scenario: An approach from bulk sensitive

photoemission study Phys Rev B 89, 121111(R) (2014).

22 Li, Q et al Atomically resolved spectroscopic study of Sr2IrO4: Experiment and theory Scientific Reports 3, 3073 (2013).

23 Wang, F et al Twisted Hubbard model for Sr2 IrO 4: Magnetism and possible high temperature superconductivity Phys Rev Lett

106, 136402 (2011).

24 Watanabe, H et al Monte Carlo study of an unconventional superconducting phase in Iridium oxide Jeff = 1/2 Mott insulators

induced by carrier doping Phys Rev Lett 110, 027002 (2013).

25 Damascelli, A et al Angle-resolved photoemission studies of the cuprate superconductors Rev Mod Phys 75, 473 (2003).

26 Ronning, F et al Anomalous high-energy dispersion in angle-resolved photoemission spectra from the insulating cuprate

Ca 2 CuO 2 Cl 2 Phys Rev B 71, 094518 (2005).

27 Graf, J et al Universal High Energy Anomaly in the Angle-Resolved Photoemission Spectra of High Temperature Superconductors:

Possible Evidence of Spinon and Holon Branches Phys Rev Lett 98, 067004 (2007).

28 Xie, B P et al High-Energy Scale Revival and Giant Kink in the Dispersion of a Cuprate Superconductor Phys Rev Lett 98,

147001 (2007).

29 Valla, T et al High-Energy Kink Observed in the Electron Dispersion of High-Temperature Cuprate Superconductors Phys Rev

Lett 98, 167003 (2007).

30 Meevasana, W et al Hierarchy of multiple many-body interaction scales in high-temperature superconductors Phys Rev B 75,

174506 (2007).

31 Chang, J et al When low- and high-energy electronic responses meet in cuprate superconductors Phys Rev B 75, 224508

(2007).

32 Inosov, D S et al Momentum and Energy Dependence of the Anomalous High-Energy Dispersion in the Electronic Structure

of High Temperature Superconductors Phys Rev Lett 99, 237002 (2007).

33 Zhang, W et al High Energy Dispersion Relations for the High Temperature Bi2Sr2CaCu2O8 Superconductor from Laser-Based

Angle-Resolved Photoemission Spectroscopy Phys Rev Lett 101, 017002 (2008).

34 Moritz, B et al Effect of strong correlations on the high energy anomaly in hole- and electron-doped high-T c superconductors

New J Phys 11, 093020 (2009).

35 Ikeda, M et al Differences in the high-energy kink between hole- and electron-doped high-T c superconductors Phys Rev B 80,

184506 (2009).

36 Byczuk, K et al Kinks in the dispersion of strongly correlated electrons Nature Phys 3, 168 (2007).

37 Manousakis, E String excitations of a hole in a quantum antiferromagnet and photoelectron spectroscopy Phys Rev B 75,

035106 (2007).

38 Leigh, R G et al Hidden Charge 2e Boson in Doped Mott Insulators Phys Rev Lett 99, 046404 (2007).

39 Markiewicz, R S & Bansil, A Dispersion anomalies induced by the low-energy plasmon in the cuprates Phys Rev B 75,

020508(R) (2007).

40 Markiewicz, R S., Sahrakorpi, S & Bansil, A Paramagnon-induced dispersion anomalies in the cuprates Phys Rev B 76, 174514

(2007).

41 Zhou, T & Wang, Z D High-energy dispersion anomaly induced by the charge modulation in high-Tc superconductors Phys

Rev B 75, 184506 (2007).

42 Macridin, A et al High-Energy Kink in the Single-Particle Spectra of the Two-Dimensional Hubbard Model Phys Rev Lett 99,

237001 (2007).

43 Zhu, L et al Universality of Single-Particle Spectra of Cuprate Superconductors Phys Rev Lett 100, 057001 (2008).

44 Tan F et al Theory of high-energy features in single-particle spectra of hole-doped cuprates Phys Rev B 76, 054505 (2007).

45 Zemljic, M M et al Temperature and Doping Dependence of the High-Energy Kink in Cuprates Phys Rev Lett 100, 036402

(2008).

46 Srivastava, P et al High-energy kink in the dispersion of a hole in an antiferromagnet: Double-occupancy effects on electronic

excitations Phys Rev B 76, 184435 (2007).

47 Alexandrov, A S & Reynolds, K Angle-resolved photoemission spectroscopy of band tails in lightly doped cuprates Phys Rev

B 76, 132506 (2007).

48 Weber, C., Haule, K & Kotliar, G Optical weights and waterfalls in doped charge-transfer insulators: A local density approximation and dynamical mean-field theory study of La2−xSrxCuO 4 Phys Rev B 78, 134519 (2008).

49 Basak, S et al Origin of the high-energy kink in the photoemission spectrum of the high-temperature superconductor

Bi 2 Sr 2 CaCu 2 O 8 Phys Rev B 80, 214520 (2009).

50 Sakai, S., Motome, Y & Imada, M Doped high-Tc cuprate superconductors elucidated in the light of zeros and poles of the

electronic Green function Phys Rev B 82, 134505 (2010).

51 Katagiri, D et al Theory of the waterfall phenomenon in cuprate superconductors Phys Rev B 83, 165124 (2011).

52 Piazza, B Dalla et al Unified one-band Hubbard model for magnetic and electronic spectra of the parent compounds of cuprate

superconductors Phys Rev B 85, 100508(R) (2012).

53 Mazza, G et al Evidence for phonon-like charge and spin fluctuations from an analysis of angle-resolved photoemission spectra

of La2−xSrxCuO 4 superconductors Phys Rev B 87, 014511 (2013).

54 Mizokawa, T & Fujimori, A Electronic structure and orbital ordering in perovskite-type 3d transition-metal oxides studied by

Hartree-Fock band-structure calculations Phys Rev B 54 5368 (1996).

55 Ge, M et al Lattice-driven magnetoresistivity and metal-insulator transition in single-layered iridates Phys Rev Lett 84,

100402(R) (2011).

X.J.Z and Y.L proposed and designed the research G.C contributed in sample preparation Y.L., L.Y., X.W.J., Y.Y.P., C.Y.C., Z.J.X., D.X.M., J.F.H., X.L., Y.F., H.M.Y., L.Z., G.D.L., S.L.H., X.L.D., J.Z., Z.Y.X., C.T.C and X.J.Z contributed to the development and maintenance of Laser-ARPES system Y.L carried out the ARPES experiment Y.L., L.Y and X.J.Z analyzed the data J.Z.Z., H.M.W., X.D and Z.F performed band structure calculations X.J.Z., Y.L and L.Y wrote the paper and all authors participated

in discussion and comment on the paper

Additional Information

Supplementary information accompanies this paper at http://www.nature.com/srep Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Liu, Y et al Anomalous High-Energy Waterfall-Like Electronic Structure in

doi: 10.1038/srep13036 (2015)

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